<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">

    <link rel="stylesheet" type="text/css" href="flex.css" />
</head>

<body>
    <div id="header">header</div>
    <div id="content" class="clearfix">
        <div class="middle">conter</div>
        <div class="left">left</div>
        <div class="right">right</div>
    </div>
    <div id="footer">footer</div>
</body>

</html>